MATERIALS FOR MCHANICAL ENGINEERING ACTUAL

MATERIALS FOR MCHANICAL ENGINEERING ACTUAL

VERIFIED SOLUTIONS.

⫸ What is the meaning of the Martensite start Ms in case of Ti alloys ?. Answer: Ms = temperature where martensitic transformation starts on cooling from β phase.Occurs in metastable β-Ti alloys (e.g., Ti-Nb, Ti-Mo, Ti-V).

Martensite forms:

α′ (hexagonal): in near-α or α+β alloys.

α″ (orthorhombic): in metastable β alloys.

Diffusionless transformation = very fast, no atomic diffusion.

Ms depends on:

Alloy composition (β-stabilizers lower Ms).Cooling rate (faster cooling → more martensite).Below Ms, β phase → α′ or α″ martensite.

Affects mechanical properties: strength, hardness, ductility.

Critical for biomedical, aerospace, and shape memory applications.

⫸ What is the Ti corrosion resistance in seawater ? X. Answer: It has a corrosion resistance higher than alloyed steels

⫸ What are the compacting methods in powder mettalurgy ? X. Answer: isostatic pressing, rolling compacting, powder pressing in rigid dies, extrusion

⫸ Which is the influence of grain size of the powders in the parts pressed in powder technology ?. Answer: Finer powders: Better sintering → higher final density, strength.Smoother surface finish.Lower green density, harder to compact.More reactive, risk of oxidation.

Coarser powders: 1 / 2

Easier to press, higher green density.Worse sintering, more porosity.Rougher surface finish.

⫸ Duplex stainless steel. Properties and characteristic. Answer: Dual-phase structure: ~50% austenite + ~50% ferrite

High strength: ~2× stronger than austenitic stainless steels (e.g., 304, 316)

Excellent corrosion resistance:

Great resistance to chloride stress corrosion cracking Good pitting and crevice corrosion resistance Good thermal conductivity, better than austenitic grades Good weldability (though not as easy as austenitic steels) Moderate toughness, lower than austenitic grades, especially at low temperatures Applications: offshore, chemical processing, desalination, heat exchangers, pressure vessels

⫸ What are Luders bands? X. Answer: They are physical and optical effects caused by localized concentration of dislocation at the component surface

⫸ What is the eriksen test ? X. Answer: It allow to get relative steel drawability

⫸ What is the mathematical relation between n and epsilon X. Answer: n = epsilon

⫸ What is the Nakashima test ? X. Answer: It allows to get the safe straining curve

⫸ How is effected the resilience test (Charpy) ? For what reason is effected ? What are the

results ?. Answer: How is it done?

A notched specimen is hit by a swinging pendulum.The test measures the energy absorbed to fracture the sample (in Joules).Performed at various temperatures to observe material behavior.Purpose (Why is it done?) To assess the toughness of a material.To determine the ductile-to-brittle transition temperature.

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